For recordable DVDs (i.e., DVD+R, etc.) a laser spot is positioned over the center of the track using a differential push-pull (DPP) signal. The signal DPP provides the relative position of the laser spot and the track center. A closed loop servo system uses the signal DPP to produce a control signal to hold the laser spot over the center of the track.
The tracks of a disc form a long spiral from the inner diameter (ID) to the outer diameter (OD) of the disc. As the disc rotates and the laser spot follows the spiral, the laser spot moves from the ID to the OD of the disc.
Referring to FIGS. 1-2; conceptual diagrams illustrating a main beam and side beam detection and position are shown. FIG. 1 illustrates a photo-diode system 10. The photo-diode system 10 includes a number of photo-diode arrays 12a-12n. The outputs of the photo-diode array 12a (when the laser beam is focused on the disc) are shown as signals E1 and E2, respectively. The outputs of the photo diode array 12b are shown as signals A, B, C and D, respectively. The outputs of the photo diode array 12n are shown as signals F1 and F2, respectively. The laser spot reflects off the disc and is imaged on the photo-diode array 10. The land-groove structure of the disc causes diffraction and the intensity between the signals A and D changes relative to the signals B and C when the laser spot moves away from a center of a track on a disc. The difference in intensity between the signals A and D due to this effect and the signals B and C of the photo detector array 12b is known as the push-pull effect.
A second effect arises when the laser does not shine through the center of the lens. In such a case, refraction through the lens causes an imaged spot on the photo detector array 12b to move to one side which leads to a changing of the relative intensity between the signals A and D versus the signal B and C. Such an effect is called the center error (or signal CE). Since it is difficult to distinguish the push-pull effect and center effects from a main beam (i.e., a signal MPP) alone, side beams (i.e., signals SPP1 and SPP2) are used. By weighting the signals SPP1 and SPP2 and subtracting the sum of the signal SPP1 and the signal SPP2 from the signal MPP, the push-pull effect and the relation position of the laser spot to the track center may be determined.
The signal MPP is defined as:MPP=(A+D)−(B+C)=TE+CE  EQ. (1)where the signals A, B, C, and D are defined as the intensity on the photo diode array 12b. A tracking error signal (or signal TE) is the push-pull effect due to the placement of the laser spot relative to the center of track. The signal CE is the center error that arises from the lens/spot offset. The side beam push-pull signal (or signal SPP) is defined as the sum of the signal SPP1 and SPP2, where:SPP1=E1−E2; andSPP2=F2−F2  EQ. (2)where the signals E1, E2, F1, and F2 are the intensity on the photo diode array 10. The signal TE is the push-pull effect due to the placement of the laser spot relative to the center of a track or a disc track center. The sign is reversed since the signal TE is offset from the center of the track by ½ track. The signal CE is generated from the offset between the lens and laser spot. The laser spots of the signals SPP1 and SPP2 have a lower intensity than the intensity of the signal MPP. Such a lower intensity is reflected by a factor K. The signal DPP is defined by:DPP=(MPP−K*SPP)=2*TE  EQ. (3)Therefore, the signal DPP does not include or depend on the signal CE.
For a given offset X, and a track pitch TP, the push-pull effect is approximately:TE=sin(2π(X/TP))  EQ. (4)
Referring to FIG. 3, a diagram illustrating a physical origin of run out on a disc is shown. The tracks on an optical disc form a long spiral. As the disc spins, the laser follows the spiral from the inner diameter (ID) to the outer diameter (OD). However, the center of the spiral is not the center of rotation of the spiral and miscentering occurs. As the disc rotates, the track moves radially relative to a fixed point (i.e., the uncontrolled laser spot). The miscentering occurs from (i) the placement of the center hole in the disc relative to track center and (ii) the placement of the motor spindle. To reduce the effect of errors by placing the laser spot on the track, a closed loop control system is used. While the closed loop control is operating, the lens will move radially with the disc so that the laser spot is held on the center of the track. Such radial motion is defined as run out. FIG. 3 illustrates various rotational angles of the disc while in the presence of run out. The rotational angle of the disc in the presence of run out is shown as 0, π/2, π and 3π/2.
Run out is proportional to center error. As the track moves radially, the lens moves radially so that the laser spot follows the center of track. The motion of the lens induces the signal CE that is synchronous to the disc rotation. The effect of the signal CE illustrates why the signal DPP is used instead of the signal MPP.
The push-pull component of the signal SPP depends on the signals SPP1 and SPP2 being exactly one half-track displaced from the signal MPP. If the signals SPP1 , SPP2 and MPP become skewed relative to the track, the signal DPP will change. Such a skewing attenuates the signal SPP. The attenuation of the signal SPP further depends on the rotational angle of the disc. Therefore, as the disc rotates while in the presence of runout, the signal SPP is modulated. The attenuation of the signal SPP leads to an attenuation of the signal DPP since the signal DPP is a function of the signal MPP and SPP.
It would be desirable to implement a system that amplifies a differential push pull signal to mitigate the effects of attenuation by predicting an attenuation factor.